This invention relates to a hand-held drill and more particularly to a hand-held drill with a self-advancing bit that advances at a controlled feed rate.
When drilling holes in certain materials, particularly graphite epoxy composite materials, the use of a hand-held drill is made difficult by the fiber breakout that occurs as the drill tip exits the material being drilled. It has been found that fiber breakout can be minimized by using carbide drills at high speeds (over 15,000 rpm) and with a controlled feed rate of the bit so that a constant pressure is maintained on the surface being drilled while preventing the drill from lunging through the material near the hole exit as the resistance to the thrust is decreased as the material being drilled is removed.
Some drills having an automatic feed rate that have previously been developed have been hampered by excessive drill runout, sometimes caused by improper alignment between the air motor housing and the outer drill housing. Also, there is a need for a smooth control over both drill advancement rate and the return motion of the drill after the drilling operation is complete.
It is therefore an object of the present invention to provide a handheld drill suitable for drilling composites which has an automatic advancing bit that advances under a controlled and sustained feed rate to minimize fiber breakout as the drill bit emerges from the backside of the hole being drilled. It is also an object of the present invention to provide such a constant-feed drill that minimizes drill runout caused by misalignments of the bit and the motor and wherein the drill bit is isolated from side-directed forces on the bit due to transfer of bending moments from the motor to the spindle. It is also an object of the present invention to provide such a drill that operates in the range above 15,000 rpm.